Two-component polyurethane coating compositions are coating compositions that crosslink by formation of urethane bonds as a result of the addition reaction between the hydroxyl groups of a hydroxyl-functional binder component and the free isocyanate groups of a polyisocyanate crosslinker component. The hydroxyl-functional binder component and the polyisocyanate crosslinker component are stored separately from one another before being mixed to form the two-component polyurethane coating composition.
Typically, aqueous two-component polyurethane coating compositions are prepared just prior to their application by mixing an aqueous hydroxyl-functional binder dispersion component with a non-aqueous hydrophobic polyisocyanate crosslinker component. Generally speaking, these components of aqueous two-component polyurethane coating compositions are miscible with one another only after a great deal of difficulty, thereby necessitating the use of equipment requiring a high input of energy. The completely mixed aqueous two-component polyurethane coating compositions ready for application are aqueous dispersions, wherein binders and polyisocyanate crosslinkers form one or more disperse phases in a continuous aqueous phase.
The optical quality of a top coating applied from an aqueous two-component polyurethane coating composition depends upon the degree of mixing of the components prior to application. It is generally known that greater homogeneity of such a mixture, for example a finer dispersion, corresponds to an increase in the quality of the coating with regard to properties, such as, appearance, leveling and gloss.
Typically, mixing equipment for two-component coatings present in automotive OEM coating plants, for example, static mixers such as Kenics mixers, do not routinely provide the necessary mixing quality for the application of aqueous two-component polyurethane coating compositions.
EP 0 685 544 A1 and U.S. Pat. No. 7,034,067 B1 disclose processes, operating on the principle of jet dispersion, for the preparation of aqueous two-component polyurethane coating compositions. Both components of the aqueous two-component polyurethane coating composition are pressed through fine nozzles at high pressure and depressurized, wherein the mixing process is characterized by the use of a dynamic mixer, which requires considerable input of energy during mixing. However, these processes have several disadvantages such as, for example, the use of elaborate mixing technology which requires large amounts of space, general preparation work for the jet dispersion devices, and considerable amounts of energy and maintenance in order to operate the jet dispersion devices.
Another possibility to facilitate mixing is to use a hydrophilized polyisocyanate crosslinker instead of a hydrophobic one. However, this may impair pot life and/or technological properties of the coating layer applied from the aqueous two-component polyurethane coating composition and cured, in particular, in case the coating layer is a top coat layer which is exposed to weather and environmental influences. In particular, in case of automotive coating applications with its high quality requirements it is expedient to use hydrophobic polyisocyanate crosslinkers in aqueous two-component polyurethane top coating compositions.
It is desirable to provide a process for the preparation of the top coat layer of an automotive OEM multi-layer coating from an aqueous two-component polyurethane top coating composition comprising an aqueous hydroxyl-functional binder dispersion component and a non-aqueous hydrophobic polyisocyanate crosslinker component without a need to use elaborate mixing technology such as dynamic mixers (i.e. nozzle jet dispersers) when mixing the components of the aqueous two-component polyurethane coating composition. In other words, it is desirable to provide such process making use of the simple and conventional technology of static mixers rather than the elaborate mixing technology of dynamic mixers but without compromising the optical quality of the finished top coat layer. Therefore, it is an object of the present invention to provide a process for the preparation of the top coat layer of an automotive OEM multi-layer coating making use of an aqueous two-component polyurethane top coating composition comprising an aqueous hydroxyl-functional binder dispersion component and a non-aqueous hydrophobic polyisocyanate crosslinker component, where the process permits a high level of top coat quality without the use of elaborate mixing technology such as dynamic mixers when mixing the components, such that the conventional multi-component static mixing installations present in automotive OEM coating plants may be used.